1. Field of the Invention
The present invention relates to a channel counter for counting the number of multiplexed channels of an optical signal in which optical signals with a plurality of mutually differing wavelengths have been wavelength division multiplexed and also to an optical amplification device for wavelength division multiplexing transmission using the same.
2. Description of the Related Art
Wavelength division multiplexing transmission is a method of increasing transmission capacity in lightwave transmission by using signals with a plurality of mutually differing wavelengths. Since this method requires lightwaves to be combined at the transmitter and divided at the receiver, loss occurs due to these combining and dividing operations. Optical amplification devices are often used to compensate for combining/dividing loss and transmission line loss.
Semiconductor optical amplifiers, Raman fiber amplifiers, Brillouin fiber amplifiers and rare-earth-doped fiber amplifiers are examples of such optical amplification devices. In particular, an erbium-doped fiber amplifier (hereinafter abbreviated to EDFA) which is a type of rare-earth-doped fiber amplifier is frequently used for the reasons that it is not polarization dependent and can be pumped by a semiconductor laser.
Generally, the output level of the EDFA is controlled so as to be constant.
In a conventional configuration of an EDFA, for instance, a portion of the wavelength division multiplexed signal is split by means of an optical coupler injected into the output portion of the EDFA and converted to electric current by a PD module. The output level is kept constant by controlling the pumping laser module by using an optical amplification controller to maintain the current at a constant value.
However, in wavelength division multiplexing transmission, not all of a number of signals determined beforehand are transmitted. In other words, the output level of each channel changes as a result of change in the number of channels of the wavelength division multiplexing signal being inputted.
A control method for limiting this change in output level is to change the output level setting in accordance with the number of channels of the multi-channel signal. A channel counter for counting the number of channels of the multi-channel signal is required in order to perform this controlling operation.
In a configuration of a conventional channel counter, a multi-channel signal inputted to the channel counter is divided into 4 by a 1.times.4 optical coupler and an optical filter module, for example, a fiber grating extracts the signal at each wavelength. A photoreceiver converts each of the signals to electric current and a channel counter counts the number of channels.
A first disadvantage of the above-mentioned conventional configuration is that the circuit configuration becomes complex. In other words, the number of splits, fiber gratings and photoreceivers must all be the same as the number of channels counted.
A second disadvantage is that an increase in the total number of channels cannot be accommodated. This is because it is not possible to count a number of channels which exceeds the number of splits.
There is an additional problem that, since only signals with wavelengths to which the optical filter is transparent can be counted, it is not possible to accommodate modifications to the wavelength placement of every channel.